The broad, long term goal ofthe proposed research is to understand the structure and function of MHC proteins and cellular factors that interact with therin. There are five specific ainns. The first aim is structural and mechanistic characterization of aminopeptidases involved in antigen processing. Recently we determined the X-ray crystal structure of ERAP1, an enzyme responsible for the final trimming of some peptide antigens so that they can be accommodated into the class I MHC binding site. In the proposed research we will extend structural studies to other aminopeptidases that have been implicated in antigen processing pathways, and we will evaluate a mechanistic model that we proposed to explain ERAPI's unusual length-dependent proteolytic activity. The second aim is to evaluate a mechanism proposed for HLA-DM action in facilitating peptide exchange from class II MHC proteins. Recently we determined the structure of HLA-DM, a catalytic peptide exchange factor, bound to HLA-DO, an inhibitor of peptide exchange. Kinetic and mutagenesis studies suggested that HLA-DO mimics the structure of an intermediate in the peptide exchange pathway. In the proposed research we will test a model for HLA-DM action, in which HLA-DM interaction with MHC induces concerted changes in the MHC 3-10 helix and adjacent extended strand region that lead to facilitated peptide exchange. The third aim is to determine the structural basis for MHC allele-specific interaction with DM and its potential as a mechanism to explain association of particular MHC alleles with autoimmunity. The class II MHC 3-10 helix.and adjacent extended strand region exhibit allele-dependent structural variation. We will test the hypothesis that this variation is related to HLADM susceptibility. The fourth aim is to understand how TCR chain pairing creates specificity for MHC-peptide complexes. How TCR are constructed to recognize a limited set of ligands is not clear. We will characterize interactions between TCR alpha subunit residues and beta subunit CDR loops, and vice versa, to determine how these shape MHC-TCR interaction. The fifth aim is to characterize the structural basis for T cell crossreactivity of epitopes from vaccina virus and lymphocytic choriomeningitis virus.